1. Field of the Invention
The present invention relates to methods for forming conductive contacts to semiconductors.
2. Description of the Prior Art
The technological development of micro-miniaturized semiconductor integrated circuit devices has required improved methods for making conductive contacts to the semiconductor impurity regions. This has been made possible by newly developed lithographic techniques, improved material deposition processes and metallurgical systems. In particular new combinations of metals have been developed for providing both ohmic as well as Schottky barrier contacts to the impurity regions. Such contacts and the methods for depositing same are described, for example, in U.S. Pat. No. 3,995,310 as well as the above-referenced patent application of Dalal et al. In general, advanced semiconductor integrated circuits require three types of contacts which are made to the impurity regions: ohmic, low-barrier-height Schottky barriers (LSB) and high-barrier-height Schottky barriers (HSB). As discussed in the Dalal et al application, and U.S. Pat. No. 3,995,301 there have been problems in selecting the correct metallurgical systems which will yield satisfactory contacts from the standpoint of switching speed, contact resistance and voltage drops.
Another problem in the formation of said contacts is the development of an optimum technique for selectively depositing a particular type of contact in one or more similar regions of the semiconductor substrate and depositing other types of contacts in other related regions.
A widely used metallurgy for providing both ohmic contacts as well as Schottky barrier contacts is a layer of a metallic silicide, typically platinum silicide, making direct contact with the silicon substrate. The reason for this is that the aluminum makes less than satisfactory contact with silicon. Deposited atop the platinum silicide may be a variety of metals, such as aluminum, chrome, gold, etc. When the platinum silicide is formed of an impurity region having a relatively low doping, a Schottky barrier diode is formed, while formation of the platinum silicide onto a high impurity concentration substrate results in an ohmic contact.
As pointed out in U.S. Pat. No. 3,995,301 platinum silicide Schottky barrier contacts have a relatively high forward barrier of around 0.8 volts. For this reason the switching speed is relatively low. It has therefore been necessary for the art to develop other metallurgical systems for forming low barrier height Schottky barrier (LSB) contacts.
One successful metallurgical system is described in the above-referenced patent application of Dalal et al. The LSB diode is formed by the deposition of tantalum under carefully controlled conditions atop an N- doped silicon semiconductor region. The HSB diode and ohmic contacts are formed first by forming platinum silicide in other N- doped and N+ doped regions, respectively. This is followed by the deposition of tantalum atop the platinum silicide.
In the process of depositing these separate layers of metals in order to form different types of semiconductor contacts, the platinum is deposited into the openings atop the substrate where the ohmic and HSB contacts are to be formed. The openings where the LSB are to be formed are protected by a diffusion mask such as silicon dioxide. After the platinum is deposited and sintered to form platinum silicide the entire substrate, with the exception of the LSB contact openings, is blocked off, typically with a photoresist mask. The diffusion mask is then etched away in the LSB contact openings. Then the photoresist mask is removed. This leaves all openings on the substrate exposed and the tantalum is then deposited in all of the openings.
This process, although resulting in satisfactory contacts, is not as reliable as desired. In the first place the deposition of the separate metallurgical systems requires the added photoresist masking step. Secondly, the layer of photoresist is deposited directly atop the platinum silicide layers. In the usual course of applying, exposing and developing the resist layer, it must be postbaked to drive out the resist solvents. This results in a tough layer which tends to adhere to the platinum silicide. Removal of the resist is quite difficult and also has been found to leave undesirable contaminants atop the platinum silicide layer.